Method and magnetic resonance system for determining the flip angle distribution in a volume of an examination subject

ABSTRACT

In a method for determination of flip angle distributions for various antenna transmission configurations in a magnetic resonance system, magnetic resonance measurements are implemented with the various transmission configurations, with the reception configuration being identical for all implemented magnetic resonance measurements, and all magnetic resonance measurements for the various transmission configurations are implemented with a specific pulse sequence. This pulse sequence is selected such that the total function that describes the dependency of the image signal at a specific location on the flip angle achieved at this location with the radiated radio-frequency field, as well as on further MR-relevant parameters, can be factored into a first sub-function that describes the dependency of the image signal on the achieved flip angle and a second sub-function (Tb) that describes the dependency of the image signal on the further MR-relevant parameters, and such that the functional dependency of the image signal on the achieved flip angle is known. The absolute flip angle distribution is measured for a reference transmission configuration, and the flip angle distributions of the other transmission configurations are then respectively determined on the basis of the absolute flip angle distribution of the reference transmission configuration and on the basis of the ratio of the spatially-dependent image signals of the magnetic resonance measurements of the respective transmission configuration to the corresponding spatially-dependent image signals of the magnetic resonance measurement of the reference transmission configuration.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention concerns a method for determining flip angledistributions for various antenna transmission configurations in atleast one specific volume region within an examination subject in amagnetic resonance system, wherein the magnetic resonance system has aradio-frequency antenna with a number of resonator elements that (indifferent transmission configurations) can be excited individually or ingroups for generation of linearly-independent radio-frequency fielddistributions in an examination volume enclosing the examinationsubject. The invention also concerns a magnetic resonance systemsuitable for implementation of such a method, with a correspondingradio-frequency antenna and a computer program product that can beloaded into a memory of a programmable control device of such a magneticresonance system for implementation of the method.

2. Description of the Prior Art

Magnetic resonance tomography, also called magnetic resonance tomographyapparatus, is a widespread technique for acquisition of images of theinside of the body of a living examination subject. In order to acquirean image with this method, the body or a body part of the patient to beexamined must initially be exposed to an optimally homogeneous staticbasic magnetic field (usually designated as a B₀ field) that isgenerated by a basic field magnet of the magnetic resonance system.During the acquisition of the magnetic resonance images,rapidly-switched gradient fields that are generated by gradient coilsare superimposed on this basic magnetic field for spatial coding.Moreover, radio-frequency pulses of a defined field strength areradiated into the examination subject with radio-frequency antennas. Themagnetic flux density of these radio-frequency pulses is typicallydesignated with B₁. The pulse-shaped radio-frequency field is thereforegenerally called a B₁ field. The nuclear spins of the atoms in theexamination subject are excited by means of these radio-frequency pulsessuch that they are deflected from their equilibrium state (parallel tothe basic magnetic field B₀) by what is known as an “excitation flipangle”(generally also called a “flip angle”). The nuclear spins thenprecess around the direction of the basic magnetic field B₀. Themagnetic resonance signals thereby generated are acquired byradio-frequency reception antennas. The reception antennas can be eitherthe same antennas with which the radio-frequency pulses are alsoradiated, or separate reception antennas. The magnetic resonance imagesof the examination subject are generated on the basis of the acquiredmagnetic resonance signals. Each image point in the magnetic resonanceimage is thereby associated with a small body volume (known as a“voxel”) and each brightness or intensity value of the image points islinked with the signal amplitude of the magnetic resonance signalacquired from this voxel. The correlation between a resonant radiatedradio-frequency pulse with the field strength B₁ and the flip angle aachieved thereby is provided by the equation

$\begin{matrix}{{\alpha = {\int_{t = 0}^{\tau}{\gamma \cdot {B_{1}(t)} \cdot {\mathbb{d}t}}}},} & (1)\end{matrix}$wherein γ is the gyromagnetic ratio which can be considered as a fixedmaterial constant for most magnetic resonance examinations, and τ is theeffective duration of the radio-frequency pulse. Aside from beingdependent on the duration of the pulse, the flip angle achieved by theemitted radio-frequency pulse (and thus the strength of the magneticresonance signal) also depends on the strength of the radiated B₁ field.Spatial fluctuations in the field strength of the excited B₁ fieldtherefore lead to unwanted variations in the acquired magnetic resonancesignal that can adulterate the measurement result.

Especially at high magnetic field strengths (that are inevitably presentdue to the necessitated basic magnetic field B₀ in a magnetic resonancetomography apparatus) the radio-frequency pulses have a non-homogeneouspenetration behavior in conductive and dielectric media such as, forexample, tissue. This leads to the situation that the B₁ field can varysignificantly within the measurement volume. In particular in theultra-high field range with magnetic field strengths B₀≧3 T, significantinfluences of the radio-frequency penetration behavior on the imagequality are observed. Due to the B₁ focusing and shielding effects, theflip angle of the radio-frequency pulses is a function of the location.Contrast and brightness of the acquired magnetic resonance imagestherewith vary in the mapped tissue and can lead, in the worst case, tothe situation in which pathological structures are not visible.

Multi-channel transmission coils, also called “transmit arrays”, arepresently considered as a promising approach to the solution of thisproblem. These are radio-frequency antennas of the aforementioned typethat have a number of resonator elements that can be activatedindividually or in groups. This is possible, for example, when theindividual resonator elements are electromagnetically decoupled from oneanother, and can be separately activated with an individual amplitudeand phase via separate radio-frequency channels. Depending on with whichamplitudes and phases the different transmission configurations (i.e.the individual resonator elements or groups of resonator elements ortransmission modes) are excited, different radio-frequency distributionsform in the examination volume of the antenna. For example, with anantenna with N resonator elements that are electromagnetically decoupledfrom one another and that can be activated individually, it is possibleto transmit in N linearly-independent transmission modes whichrespectively lead to different field distributions.

A simple example of this is a birdcage resonator, that has rods that areactivated individually with regard to amplitude and phase. Each of theserods generates a B₁ field independent of one another, with the B₁ fieldsof the individual rods overlapping to form the total field distribution.Instead of individually considering the single resonator rods, different“collective excitation modes” of the entire birdcage antenna can also beindividually excited. One of these modes is, for example, the standardexcitation mode of the birdcage resonator (known as the “CP mode”) inwhich the radio-frequency voltage from rod-to-rod varies in terms of thephase by 360°/N_(R) (wherein N_(R) is the number of the rods). Forhigher order collective modes, the voltage then varies, for example, by2·360°/N_(R), 3·360°/N_(R), etc. from rod to rod. For activation of suchcollective modes, a resident-power mode matrix is installed, forexample, in the hardware used for the activation of the antennaelements.

Through individual settings of the amplitude and the phase of theradio-frequency pulse radiated by each transmission configuration (i.e.by each transmission element or each transmission mode), the spatialdistribution of the B₁ field can be influenced with the goal to generatean optimally homogeneous radio-frequency field in the subject or,respectively, in the examination volume. Magnetic resonance systems ofthis type are, for example, specified in U.S. Pat. No. 6,043,658 and DE10 2004 045 691 A1.

A significant, and as of yet largely unresolved, step in this context isthe determination of the optimal individual transmission parameters,i.e. of the transmission amplitudes and transmission phases for theindividual transmission configurations. Previous approaches fordetermination of optimized transmission parameters have been basedeither on simulations, such as in DE 10 2004 045 691 A1. For thispurpose, a body model is required in order to simulate theradio-frequency penetration behavior and to be able to calculate therequired parameters. A subject-specific optimization (i.e. anoptimization that is directly adapted to the current examinationsubject), however, is therewith not possible. Alternatively, theabsolute B₁ distribution of every single transmission element can bemeasured per slice. This does in fact allow an object-dependentadjustment, but these measurements are extraordinarily time-consuming.Approximately 10 sec. per transmission configuration and per sliceacquisition are presently required. For six transmission configurationsand 10 slices, this leads to an adjustment time of 10 minutes. Thismethod is thus not practical in practice as an adjustment method.

SUMMARY OF THE INVENTION

An object of the present invention is to allow the radio-frequency fielddistributions that occur with a wide variety of transmissionconfigurations in magnetic resonance tomography to be quickly determinedso that the optimal, individual parameters for subsequent MRmeasurements can be determined on the basis of these values.

In accordance with the invention for determination of theradio-frequency distribution for the various transmissionconfigurations, this object is achieved by implementing the followingsteps (with the order of the first two steps being arbitrary):

1) Magnetic resonance measurements are implemented to generate magneticresonance images of the appertaining volume region of the examinationsubject with the various transmission configurations. The receptionconfiguration of the reception antenna (which can be the same antenna asthe transmission antenna) for measurement of the image signals arisingfor the magnetic resonance measurements is identical in all implementedmagnetic resonance measurements. Moreover, all magnetic resonancemeasurements for the various transmission configurations are implementedwith a specific pulse sequence, which is selected such that the functionthat generally specifies the dependency of the image signal at aspecific location on the flip angle achieved at this location with theradiated radio-frequency field, as well as on further MR-relevantparameters (such as the local spin density or the relaxation times) canbe factored into at least two sub-functions. The first sub-function thatdescribes the dependency of the image signal on the achieved flip angleand the second sub-function that describes the dependency of the imagesignal on the further MR-relevant parameters. Additionally, the pulsesequence must be selected such that the functional dependency of theimage signal on the achieved flip angle (i.e. the first sub-function) isknown.

2) An absolute flip angle distribution in at least one specific volumeregion within the examination subject is measured for a “referencetransmission configuration”. Suitable methods for this are known tothose skilled in the art, for example from United States PatentApplication Publication Nos. 2005/0073304 A1 and 2004/0164737 A1. Thereference transmission configuration can be an arbitrary transmissionconfiguration selected from the possible linearly-independenttransmission configurations; the CP mode is advantageously utilized as areference transmission configuration.

3) The flip angle distributions of the other transmissionconfigurations, i.e. of the transmission configurations which do notcorrespond to the reference transmission configuration, are thenrespectively determined on the basis of the absolute flip angledistribution of the reference transmission configurations and on thebasis of the ratio of the spatially-dependent image signals of themagnetic resonance measurements of the respective transmissionconfiguration to the corresponding spatially-dependent image signals ofthe magnetic resonance measurement of the reference transmissionconfiguration. For the determination of the absolute flip angledistributions of the various transmission configurations, it issufficient to consider the mathematical ratio of the amplitudes of thecomplex image signals measured with the appertaining transmissionconfiguration to the amplitudes of the complex image signals measuredwith the reference transmission configuration.

The flip angle α measured at a specific location is representative forthe B₁ field radiated at that location, the dependency being provided byequation (1). This means that an arbitrary conversion can be made from aflip angle distribution into a B₁ field distribution and vice versausing this equation (given knowledge of the employed pulse). The terms“flip angle distribution” and “B₁ field distribution” and“radio-frequency field distribution” are therefore used synonymouslyherein. The determination of a radio-frequency field distribution isaccordingly also equated with a determination of the corresponding flipangle distribution and vice versa in the sense of the explanationherein.

The invention is based on the essential realization that the measurementof the absolute B₁ amplitude or of the flip angle is only necessary fora reference transmission configuration, i.e. for one transmissionelement or one transmission mode. For all other transmissionconfigurations, the determination of the B₁ amplitude or of the flipangle a relative to the reference transmission configuration issufficient. The method is thereby significantly accelerated.

Mathematically, the mode of operation of the inventive method can beexplained as follows:

The amplitude S^(A) of the complex image signal S at a location r (rbeing a vector) is generally linked by the sensitivity profile R of thereception coil and the flip angle a according to the following equation:S ^(A)(r)=R(r)T(α(r), f(r))  (2)

The parameter f here encompasses all MR-relevant parameters, for examplethe local spin density, the relaxation times T₁ and T₂ etc. The functionT generally describes the complex dependency of the generatedmagnetization on these parameters as well as on the flip angle α, whichshould be treated separately. In general, more than one radio-frequencypulse or flip angle can naturally also be used in a sequence, but thisdoes not alter the equation since the spatial distribution of the pulseamplitude in the subject is always identical.

If a sequence is now selected for the measurement in accordance with theinvention such that a factorization of the total function T is possible,this can be mathematically expressed as follows:T(α(r), f(r))=T _(a)(α(r))·T _(b)(f(r))  (3)

For example, relaxed spin echo or gradient echo sequences (i.e. spinecho or gradient echo sequences in which the repetition time T_(R) ismuch larger than the relaxation time T₁) belong to the pulse sequencesin which such a factorization of the total function T into twosub-functions T_(a) and T_(b) is possible. Sequences known as EPIsequences (EPI=echo planar imaging), in particular FID-EPI sequences(FID=free induction decay) or SE-EPI sequences (SE=spin echo) canlikewise be used. A significant advantage of these acquisitiontechniques is the short acquisition duration. Still more imagingtechniques known to those skilled in the art are likewise suitable, suchas, for example, segmented EPI or sequences known as spiral sequences.

Moreover, according to the invention it must be heeded that thesub-function T_(a)(α(r) must be known. This is fortunately the case inmany sequences. For example, given a relaxed gradient echo sequence orgiven a FID-EPI sequence:T _(a)(α(r))=sin(α(r))  (4)

Given a relaxed spin echo sequence or given an SE-EPI sequence with thepulse scheme “α-β-echo” (with the excitation flip angle α and therefocusing flip angle β), it applies that:

$\begin{matrix}\begin{matrix}{{T_{a}\left( {{\alpha(r)},{\beta(r)}} \right)} = {\frac{1}{2} \cdot {\sin\left( {\alpha(r)} \right)} \cdot \left( {1 - {\cos\left( {\beta(r)} \right)}} \right)}} \\{= {{\sin\left( {\alpha(r)} \right)} \cdot {\sin^{2}\left( \frac{\beta(r)}{2} \right)}}}\end{matrix} & (5)\end{matrix}$

If an image is now acquired in accordance with the invention for eachtransmission configuration i (inclusive of the transmissionconfiguration i=k) with the selected sequence,S _(i) ^(A)(r)=R(r)·T _(a)(α_(i)(r))·T _(b)(f(r))  (6)is obtained for each transmission configuration.

This same equation also applies for the reference transmissionconfiguration (with i=k), for which the flip angle distribution α_(k)(r)was measured once in accordance with the invention.

The ratios of the amplitudes of the image signals of the individualtransmission configurations i≠k to those of the reference transmissionconfiguration k are consequently independent of reception profile and ofthe local MR-relevant parameters:

$\begin{matrix}{\frac{S_{i}^{A}(r)}{S_{k}^{A}(r)} = \frac{T_{a}\left( {\alpha_{i}(r)} \right)}{T_{a}\left( {\alpha_{k}(r)} \right)}} & (7)\end{matrix}$

The absolute flip angle α_(i)(r) of each transmission configuration i≠kthus can be determined as follows from the independently-determinedabsolute flip angle α_(k)(r) of the reference configuration (k) and themeasured image signals as well as the correlation known according toequation (7):

$\begin{matrix}{{\alpha_{i}(r)} = {T_{a}^{- 1}\left( {{T_{a}\left( {\alpha_{k}(r)} \right)} \cdot \frac{S_{i}^{A}(r)}{S_{k}^{A}(r)}} \right)}} & (8)\end{matrix}$wherein T_(a) ⁻¹ designates the inversion of the function T_(a), thus(for example) T_(a) ⁻¹=arcsin for T_(a)=sin. The flip angledistributions for all other transmission configurations can thus becalculated very quickly by means of equation (8).

In addition to a radio-frequency antenna described above with a numberof resonator elements (that can be activated individually or in groups)as well as an antenna activation device in order to excite the resonatorelements individually or in groups in different transmissionconfigurations for generation of linearly-independent radio-frequencydistributions in an examination volume enclosing an examination subject,an inventive magnetic resonance system must include the followingfurther components:

-   -   A measurement sequence control unit that initiates the        implementation of magnetic resonance measurements with the        various transmission configurations for determination of flip        angle distributions for various transmission configurations in        at least one specific volume region within an examination        subject. As described above, the reception configuration of the        reception antenna is identical for all magnetic resonance        measurements; and wherein all magnetic resonance measurements        for the various transmission configurations are implemented with        the pulse sequence suitably selected according to the above        explanation.    -   A flip angle distribution measurement unit in order to determine        the absolute flip angle distribution in at least the specific        volume region within the examination subject for a reference        transmission configuration. For this purpose, the flip angle        distribution measurement unit can advantageously influence the        measurement sequence control unit insofar as that suitable        measurement sequences are radiated for measurement of the        absolute flip angle distribution.    -   A flip angle calculation unit which, on the basis of the        absolute flip angle distribution of the reference transmission        configurations and on the basis of the ratio of the        spatially-dependent image signals of the magnetic resonance        measurements of the other transmission configurations,        respectively determines the flip angle distribution of the other        transmission configurations relative to the corresponding,        spatially-dependent image signals of the magnetic resonance        measurement of the reference transmission configuration.

The antenna activation device, the measurement sequence control unit,the flip angle distribution measurement unit and the flip angledistribution calculation unit are advantageously integrated at least inpart into the typical system control device that moreover is also usedto control the magnetic resonance system. The antenna activation device,the measurement sequence control unit, the flip angle distributionmeasurement unit and the flip angle distribution calculation unit alsocan be fashioned in multiple parts, i.e. various modules that forexample, are integrated into different components of the system controldevice. The realization advantageously ensues in the form of one or moresoftware modules that can be called as an antenna control program, ameasurement sequence control program, a flip angle distributionmeasurement program or a flip angle distribution calculation program,within a computerized control device of the magnetic resonance system.As used herein a “computerized control device” means a control devicethat is equipped with a suitable processor as well as further componentsin order to execute the control, measurement and/or calculation programsthat are provided.

Since, as described above using equation (8), the inverse function T_(a)⁻¹ of the function T_(a)(α(r)) is used that describes the dependency ofthe image signal S(r) on the flip angle α(r), in the implementedmagnetic resonance measurements in the various transmissionconfigurations it should be insured that nowhere in the generated imagesdoes a local flip angle α(r) result outside of the region in which theinverse function T_(a) ⁻¹ is unique. For the experiments in which, forexample, the functional correlation results according to equation (4) bya simple sine function (as given a simple relaxed gradient echo sequenceor an FID-EPI sequence), it should be ensured that the flip angle α(r)in the acquired MR images is maximally 90°. An estimation for themeasurement flip angle normally can be obtained in advance withoutproblems, so that it can be ensured that the flip angle always remainswithin the unambiguous range. For this purpose, for example, the simpleand fast methods described in United States Patent ApplicationPublication 2005/0073304 A1 can be used for determination of an averageB₁ field within a slice to be measured by means of a double echo method,without spatial resolution. If the average B₁ field is known and it isalso assumed that the maximum B₁ field within a slice exceeds theaverage by not more than a specific factor, for example a factor of 2,the measurement flip angle can be concluded and a suitable field can beradiated for the magnetic resonance measurement, such that the flipangle safely remains in the unambiguous range. For example, for anFID-EPI measurement, and an SE-EPI measurement the excitation flipangles in the measurements can be set to 45° in order to make sure thatthe maximum flip angle is no more than 90°.

As described above, the optimal phase for the individual transmissionconfigurations is required as a further transmission parameter. Todetermine the optimal transmission parameters, the phasing of theradio-frequency distribution of the various transmission configurationsrelative to one another are advantageously determined on the basis ofthe phasing of the image signals that were determined in the implementedmagnetic resonance measurements with the various transmissionconfigurations. This is very simple in the inventive method since therequired phasing φ_(i)(r) (related to the reference phase positionφ_(k)(r)) can be automatically determined from the phase of the imagesignals S_(i)(r) and S_(k)(r). It is only important that in both cases(as is inventively the case anyway) the same reception configuration isused, such that the reception phases have no influence. Furthermore, thecorrelation of the phasing of the RF pulse or, respectively, RF pulsesand of the detected magnetic resonance signal must be known. For asimple gradient echo, for example an FID-EPI sequence,φ_(i)(r)−φ_(r)(r)=Arg(S _(i)(r))−Arg(S _(k)(r))  (9)wherein S(r) is the complete, complex image signal and the Arg functionextracts the phase from this image signal. Similar correlations arefound in the spin echo sequences, independent of which radio-frequencypulse is individually applied with the respective transmissionconfiguration i.

For this purpose, a magnetic resonance system in accordance with theinvention requires a suitable phase determination unit that, forexample, can be a part of the flip angle calculation unit.

Insofar as multi-pulse sequences (for example spin echo sequences) areused for implementation of the magnetic resonance measurement with thevarious transmission configurations, advantageously only one pulse ofthe sequence is emitted with the appertaining transmission configurationand the other pulses of the sequence are emitted with the referencetransmission configuration. The signal/noise ratio can thereby beimproved. For a multi-pulse sequence with an excitation pulse and arefocusing pulse, the excitation pulse is preferably emitted with theappertaining transmission configuration and the refocusing pulse isemitted with the reference transmission configuration in order to avoidphase ambiquities.

To accelerate the method, the magnetic resonance measurements arepreferably implemented in an interleaved multi-slice acquisition method(known as an “interleaved” measurement method). In such an interleavedslice acquisition mode, the measurement time in the ideal case can bereduced to the fraction 1/N_(s)(N_(s)=number of the slices). In such aslice measurement, in order to avoid the problem that the signals resultas integrals over the flip angle distribution in the respective slicealong the slice normals, multi-pulse sequences with at least oneexcitation pulse and one refocusing pulse are advantageously used forthese measurements. One of the pulses is emitted slice-selectively andthe other pulse is emitted weakly slice-selectively, for example withdouble the slice width. The refocusing pulse is preferably the pulsethat is emitted slice-selectively.

The distribution along the slice normals that is relevant for the flipangle determination is therewith significantly reduced. Nevertheless,multi-slice acquisitions are possible. This method can also beadvantageously used in the one-time determination of the absolute flipangle for the reference transmission configuration.

Moreover, the inventive measurement allows the generation of ahomogeneous radio-frequency distribution in at least one specific volumeregion within an examination subject in a magnetic resonance system; themagnetic resonance system having a radio-frequency antenna with a numberof resonator elements that can be excited individually or in groups forgeneration of linearly-independent radio-frequency distributions in anexamination volume in which the examination subject is located. For thispurpose, according to the inventive method described above only the flipangle distribution in the specific volume region must be initiallydetermined for the various transmission configurations. A determinationof the optimal transmission amplitudes for the individual transmissionconfigurations can subsequently ensue on the basis of the determinedflip angle distribution. This means that a radio-frequency pulseexcitation profile in which the radio-frequency field distributionexhibits a specific shape is determined on the basis of the measuredflip angle distribution or radio-frequency field distributions. Theexcitation in a later MR measurement then ensues according to thedetermined radio-frequency pulse excitation profile.

For this purpose the inventive magnetic resonance system has a parameteroptimization unit that, on the basis of the determined flip angledistribution, determines the optimal transmission amplitudes for theindividual transmission configurations for generation of a homogeneousmagnetic radio-frequency field distribution in at least the specificvolume region of the examination subject, on the basis of which theantenna activation device activates the resonator elements in asubsequent magnetic resonance measurement.

The inventive method for generation of a homogeneous magneticradio-frequency distribution can be used both in an activation ofindividual resonator elements and in a group-by-group activation of theresonator elements in established transmission modes, for example bymeans of a resident-power mode matrix. For simplicity, however, in thescope of the invention an activation of the individual resonatorelements is primarily discussed for the most part which, is to beequated with an activation of groups of resonator elements that arecoupled via a corresponding mode matrix.

Moreover, for the various transmission configurations the phasepositions of the radio-frequency field distributions of the varioustransmission configurations are also advantageously determined relativeto one another as previously described, and a determination of theoptimal transmission phases for the individual transmissionconfigurations then ensues on the basis of the determined phasepositions. The parameter optimization unit must be correspondinglyfashioned for this purpose so that it can determine the optimaltransmission phases for the individual transmission configurations onthe basis of the determined phase positions.

One possibility for calculation of the optimal transmission parametersfor individual transmission configurations from the determined B₁ fielddistribution or the flip angle distribution and the determined phasepositions is described by D. Diehl, U. Weinert, E. Bijick, R. Lazar andW. Renz in “B₁ Homogenization at 3T MRI Using a 16 Rung Transmit Array”,Proceedings of the ISMRM, 13th Meeting, Page 2751, 2005. For example, ifone is limited to the minimization of the homogeneous function f₁ (forexample the standard deviation of B₁ within the examination volume),only the spatial distribution of the B₁ fields generated by theindividual transmission configurations is required as input information.This information can be acquired from FDTD simulations with a bodymodel, but information acquired from a measurement obviously representsthe better database due to the strong dependency of the B₁ fielddistribution on the detailed body structure and geometry. The inventivemethod enables the acquisition of corresponding measurement data. Thecalculation of the optimal amplitudes and phases of the currents I₁, I₂,. . . I_(N) of the individual transmission configurations then turns outto be a standard optimization problem that can be solved by thoseskilled in the art with known methods from the numerical mathematics.The simultaneous consideration of a second optimization function f₂ withwhich, for example, the SAR is kept optimally low, is likewise possible.

By the use of the inventive method a subject-specific optimization ofthe transmission parameters of a transmit array within an acceptabletime span is now possible for the first time. Given use of fasteracquisition techniques (such as, for example, an EPI method), the timeexpenditure for the adjustment method can lie in the range of less than20 sec. inclusive of the determination of the absolute reference flipangle.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart of an embodiment of the inventive method.

FIG. 2 is a pulse sequence for parallel, spatially-resolved measurementof a number of slices.

FIG. 3 a illustrates a slice-selective excitation pulse and a subsequentslice-selective refocusing pulse on the time axis (upper diagram) andthe associated slice profile, i.e. the flip angle as a function of thelocation along the slice selection axis z (lower diagram).

FIG. 3 b illustrates a non-slice-selective excitation pulse and of asubsequent slice-selective refocusing pulse on the time axis (upperdiagram) and the associated slice profile, i.e. the flip angle as afunction of the location along the slice selection axis z (lowerdiagram).

FIG. 3 c illustrates a weakly slice-selective excitation pulse and of asubsequent slice-selective refocusing pulse on the time axis (upperdiagram) and the associated slice profile, i.e. the flip angle as afunction of the location along the slice selection axis z (lowerdiagram).

FIG. 4 schematically illustrates an inventive magnetic resonance systemin accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A possible measurement and calculation process according to anembodiment of the inventive method is shown in FIG. 1 using a workflowplan.

In step I the absolute flip angle distribution α_(k)(r) in theexamination volume is thereby initially measured with the referencetransmission configuration k with a three-dimensional or multi-slicetwo-dimensional method. This can ensue, for example, with the methoddescribed in United States Patent Application Publication 2005/0073304A1 or 2004/0164737 A1. Subsequently, a control variable i is initiallyset to 1 in step II. In a subsequent loop the factorizablethree-dimensional or multi-slice two-dimensional measurements are thenrespectively implemented with all transmission configurations i=1through N (as already described above). The result are [sic] magneticresonance images, wherein the spatial image signal distribution can bedesignated as S_(i)(r). The same reception configuration should therebyrespectively be selected.

The control variable i is incremented by 1 in step IV and in step V thequery then ensues as to whether the control variable i has reached thenumber of the possible transmission configurations N. If this is not thecase, in a step III for the new transmission configuration i+1 a newimage is acquired in order to obtain the distribution S_(i+1)(r). Instep IV the variable is then again incremented by 1.

If it is established in step V that the magnetic resonance measurementshave been conducted for all N possible transmission configurations, thecontrol variable i is initially reset to 1 (step VI).

Subsequently, in a further loop which comprises the method steps VII,VIII and XI the absolute flip angle distribution in the examinationvolume is respectively initially calculated with the transmissionconfiguration i according to the equation (9). In step IX thecalculation of the relative B₁ phase position in the examination volumeof the transmission configuration i subsequently ensues corresponding toφ_(i)(r)=f(S_(i)(r), S_(k)(r)).

In step IX the control variable i is again increased by 1 and in step Xit is queried whether the loop has already been run through for alltransmission configurations i=1 through N. Otherwise a return to themethod step VII ensues. If it is established in step X that the controlvariable i corresponds to the number N of the maximumlinearly-independent transmission configuration, the method is ended inthe step XI.

On the basis of the flip angle distributions and phase positions soacquired for the individual transmission configurations, the optimaltransmission parameters for activation of various receptionconfigurations (for example of the individual resonator elements) cansubsequently be determined very quickly with the methods already known,such that a homogeneous B₁ field can be generated within the relevantvolume of the examination subject.

In the pulse sequence scheme shown in FIG. 2 the radio-frequency pulsesemitted by the radio-frequency transmission antenna and the variousgradient pulses (temporally dependent on the radio-frequency pulses)appropriately switched are shown in a typical manner on parallel timeaxes.

The radio-frequency pulses emitted by the radio-frequency transmissionantenna are shown on the upper three axes with the designations RF₁,RF₂, RF₃ (RF=radio frequency). The upper axis RF₁ thereby shows theemission of the pulses with a first transmission configuration, thesecond axis RF₂ shows the emission of pulses with a second transmissionconfiguration, and the third axis RF₃ shows the emission of the pulseswith a third transmission configuration. Here the first transmissionconfiguration is adopted as a reference transmission configuration, butin principle any other transmission configuration can be selected as areference transmission configuration. Naturally the pulse schemeaccording to FIG. 2 can be arbitrarily expanded [extended] by stillfurther transmission configurations.

The gradient GS shown below this is known as the slice-selectiongradient which, for example, is applied in the z-direction and providesfor a selection of a specific slice upon excitation of the spins.Located below this is as the phase coding gradient (phase encodinggradient) GP which provides for a phase coding. This phase codinggradient is very quickly switched through various values during ameasurement. The third gradient GR is what is known as the readout orfrequency encoding gradient which is applied in order to read outfrequency-coded signals in a specific slice. Overall, aspatially-resolved measurement of signals within the slice determined(as well) by the slice-selection gradient GS can ensue via correspondingswitching of the phase coding gradient GP and readout gradient GR. Theprecise workflow of the phase coding and frequency coding forspatially-resolved measurement within a slice as well as therepresentation in such a sequence scheme are known to those skilled inthe art and are therefore not explained in further detail here. Thesignal actually measured by the ADC (analog-digital converter) is shownon the lowermost time axis.

An SE-EPI radio-frequency pulse sequence is shown concretely in FIG. 2.For spatially-resolved measurement a weakly slice-selective firstexcitation pulse RFA₁ is initially emitted via the radio-frequencyantenna, which first excitation pulse RFA₁ provides that the spins aretilted by a specific flip angle in a precisely defined slice, forexample perpendicular to the field direction of the B₀ field(z-direction). The slice selection is achieved in that, on the one hand,a gradient pulse GSP_(A) acting in the z-direction is emitted parallelto the emission of the radio-frequency pulse RFA₁ and the frequency andthe shape of the excitation pulse RFA₁ is selected in a suitable manner.The precise effect of such a weakly slice-selective pulse issubsequently explained in detail using FIGS. 3 a through 3 c.

A short negative gradient pulse GRP follows the first slice gradientpulse GSP_(A) (which is required for slice selection for the excitationpulse RFA₁) in order to again reset (rephasing pulse) the unwanteddephasing of the magnetization that is inevitably generated by the slicegradient pulse GSP_(A).

The second pulse within such a pulse sequence PS₁ is a refocusing pulseRFR₁ which is emitted slice-selectively. A further gradient pulseGSP_(R) is emitted in order to achieve the slice selection.

Pre-dephasing pulses KDP, VDR are set in the phase coding gradient GPand in the readout gradient GR parallel to the rephasing pulse GRP inthe slice gradient GS.

After the refocusing pulse RFR₁ has been set, a series of, for example,triangular phase coding pulses PKP is set after the provided time in thephase coding gradient GP, wherein at the same time an alternatingreadout gradient pulse AGP is set in the readout gradient GR. The signalis read parallel to this at the ADC.

This pulse sequence PS₁ for the acquisition in the first transmissionconfiguration i=1 can then be repeated for each individual slice. Inorder to reduce the time duration for the measurement of a large volume,however, a number of radio-frequency excitation pulses can also beemitted under switching of a corresponding slice-selective gradient GSin an interleaved manner. The implementation of such a multi-slicemethod is known to the average man skilled in the art and does not needto be explained here in detail. However, it should be insured that insuch a multi-slice measurement it is ensured that, on the one hand, theindividual pulses do not act on the entire volume (meaning that slicesare excited that should not be excited in a specific slice measurement)and that, on the other hand, the influence of the flip angledistribution along the slice normals does not adulterate themeasurements. FIGS. 3 a through 3 c are referenced to explain thisproblem.

An excitation pulse RFA (solid line) is shown on the upper axis RF inFIG. 3 a and a refocusing pulse RFR (dashed line) is subsequently shown.Both pulses RFA, RFR are slice-selective pulses, for example in that asinc function pulse shape is selected. It is assumed that a matchingslice gradient pulse is simultaneously emitted. In the graphic arrangedbelow these the slice profiles (i.e. the flip angle α) are plotted as afunction of the z-coordinate, wherein here z represents theslice-selection axis. The boxes deposited with grey show the desired orideal rectangular slice profile for a multi-slice sequence workflow.Here the slices each have a thickness d and an interval a from oneanother. The solid black line contrarily shows the slice profile excitedwith the excitation pulse RFA and the dashed line shows the sliceprofile excited by the refocusing pulse RFR. As FIG. 3 a shows, both theexcitation pulse RFA and the refocusing pulse RFR are actuallyselective. The advantage is that both the excitation pulse RFA and therefocusing pulse RFR of a slice do not influence the neighboring slices.Slices situated next to one another can therewith be rapidly measured insuccession. The disadvantage, however, is that the flip angle is notconstant over the slice to be measured, which makes a quantitativeevaluation massively more difficult.

FIG. 3 b shows a case in which the excitation pulse RFA is notslice-selective. A rectangular excitation pulse RFA (see upper graphic)is used for this. Only the refocusing pulse RFR is slice-selective (asin FIG. 3 a). As the solid line shows in the lower graphic forrepresentation of the slice profiles, the flip angle of the excitationpulse is then constant over the slice to be measured. The neighboringslices are disadvantageously also encompassed by the excitation pulse,such that no fast repetition of measurements of a plurality of slices ispossible.

FIG. 3 c shows a further variant according to a preferred exemplaryembodiment of the present invention. Here a “weakly-selective”excitation pulse RFA is emitted. The refocusing pulse RFR is againslice-selective for slice localization, as in FIGS. 3 a and 3 b. As thesolid line in the lower graph of FIG. 3 c shows, the weakly-selectiveexcitation pulse RFA results in the neighboring slices not beinginfluenced, but at the same time the flip angle of the excitation pulseRFA is nearly constant across the slice. The precise shape of theexcitation pulse must be selected dependent on the interval a betweentwo slices. A slice interval a which approximately corresponds to theslice thickness d is by all means reasonable for the inventivemeasurements to be implemented.

As FIG. 3 c shows, in the inventive method it is thus possible withoutfurther measures to conduct the measurements very quickly in amulti-slice method by suitable selection of a weakly slice-selectiveexcitation pulse RFA and a subsequent slice-selective refocusing pulseRFR.

The emission of the pulse sequence PS₂ for the magnetic resonancemeasurement with the second antenna configuration is shown in FIG. 2. Asa comparison with the first pulse sequence PS₁ shows, this measurementdiffers merely in that the weakly slice-selective excitation pulse RFA₂is emitted here with the second transmission configuration. Theassociated slice-selective refocusing pulse RFR₂ is emitted precisely asin the first pulse sequence PS₁ with the reference configuration i=1.The switching of the slice gradients, the phase coding gradients and thereadout gradients GR also does not differ from the pulse sequence PS₁for measurement of the first transmission configuration.

The same analogously applies for the pulse sequence PS₃ for the thirdtransmission configuration. Here as well only the excitation pulse RFA₃with the third transmission configuration i=3 is emitted, and theassociated refocusing pulse RFR₃ is emitted with the first transmissionconfiguration.

As described above for the first transmission configuration, the pulsesequences PS₂, PS₃ for magnetic resonance measurement of the secondtransmission configuration and the third transmission configuration canbe developed for implementation of a multi-slice method.

FIG. 4 shows a simple basic block diagram of an exemplary embodiment ofa magnetic resonance apparatus 1 with which the inventive method can beimplemented.

The core of this magnetic resonance system 1 is a data acquisitiondevice 3 (also called a “scanner”) in which is positioned a patient O ona bed 8 in an annular basic field magnet. A radio-frequency antenna 6for emission of the MR radio-frequency pulses is located within thebasic field magnet.

The antenna 6 has N resonator elements 7 that can be individuallyactivated with radio-frequency pulses RF(A₁, θ₁), RF(A₂, θ₂), . . . ,RF(A_(N), θ_(N)). For example, this can be an antenna design asdescribed in U.S. Pat. No. 6,043,658 or in DE 10 2004 045 691 A1.

The data acquisition device 3 is activated by a system control device 2,shown separately. A terminal 4 as well as a bulk storage 5 are connectedto the system control device 2. The terminal 4 serves as a userinterface via which an operator operates the system control device 2 andtherewith the data acquisition device 3. The bulk storage 5 serves tostore images acquired, for example, by means of the magnetic resonancesystem. The terminal 4 and the storage 5 are connected with the systemcontrol device 2 via an interface 16.

The system control device 2 has an antenna activation device 9 that isconnected with the tomograph 3 and which outputs the radio-frequencypulses with the suitable amplitudes and phases for the individualresonator elements 7 corresponding to the measurement sequence protocolprovided by the system control device 2.

Moreover, the system control device 2 is connected with the dataacquisition device 3 via an interface 10. The measurement data comingfrom the tomograph 3 are acquired via the interface 10 and assembledinto images in a signal evaluation unit 11, which images are then (forexample via the interface 16) displayed at the terminal 4 and/or storedin the storage 5.

Both the system control device 2 and the terminal 4 and the storage 5can be integral components of the data acquisition device 3. The systemcontrol device 2 can be formed of a number of individual components. Forexample, the antenna activation device 9 can be fashioned as a separateunit connected with the system control device 2 via a suitableinterface.

The entire magnetic resonance apparatus 1 also includes all furthertypical components or features such as interfaces for connecting to aconnection network (for example an image information system (picturearchiving and communication system, PACS). For clarity, these componentsare not shown in FIG. 4.

The operator can communicate with the measurement sequence control unit13 in the system control device 2 via the terminal 4 and the interface16. This measurement sequence control unit 13 then provides suitablepulse sequences to the antenna activation device 9 and a gradient coilactivation device 17 with which the gradients are appropriatelycontrolled. This means that the measurement sequence control unit 13provides for an emission of the matching radio-frequency pulse sequencesvia the antenna 6 and for a suitable switching of the gradients in orderto implement the desired measurements.

Here the inventive magnetic resonance apparatus 1 has flip angledistribution measurement unit 12 as part of the system control device 2.According to a corresponding command via the terminal 4 and/or whollyautomatically within an examination program workflow, this flip angledistribution measurement unit 12 provides that the emission of pulsesequences for measurement of an absolute flip angle distribution isinitially initiated for a reference transmission configuration of theantenna 6. The magnetic resonance signals thereby measured are in turnpassed from the signal evaluation unit 11 to the flip angle distributionmeasurement unit 12 which correspondingly evaluates these in order todetermine the absolute flip angle distribution.

Moreover, the measurement sequence control unit 13 inventively allowsmagnetic resonance exposures to be generated for all transmissionconfigurations i=1 through N, as explained in detail above. The imagesignals thereby arising are likewise accepted by the signal evaluationunit 11 and the magnetic resonance images are passed to a flip anglecalculation unit 14. This flip angle calculation unit 14 moreoverreceives from the flip angle distribution measurement unit 12 theabsolute flip angle distribution α_(k)(r) for the reference transmissionconfiguration k. According to the inventive method, the absolute flipangle distribution for all transmission configurations i=1 through N canbe determined within the flip angle calculation unit.

Moreover, using the magnetic resonance measurements the relative phasingof the radio-frequency field distributions in the various transmissionconfigurations relative to one another is determined in a phasedetermination unit 14 a which here is a sub-module of the flip anglecalculation unit 14.

These values, the determined flip angle distributions and the determinedrelative phasings, are then passed to a parameter optimization unit 15which, on the basis of these values, determines the optimal amplitudesA₁, A₂ . . . A_(N) and the optimal phases θ₁, θ₂, . . . , θ_(N) for theindividual resonator elements 7 for a subsequent magnetic resonancemeasurement. The optimal transmission parameters are then passed to themeasurement sequence control unit 13 that provides that the antennaactivation device 9 correspondingly activates the antenna elements 7.

At least the measurement sequence control unit, the flip angledistribution measurement unit 12, the signal evaluation unit 11, theflip angle calculation unit 14 and the parameter optimization unit arenormally realized in the form of software modules on a processor of thesystem control device 2. The realization purely as software has theadvantage that already-existing magnetic resonance systems can also beupgraded (retrofitted) via a corresponding software upgrade. It is alsopossible for the units 11, 12, 13, 14, 15 (respectively represented asindividual blocks in FIG. 4), or corresponding software modules, are inturn each formed by a multiple components or sub-routines. Thesesub-routines can also be used by other components of the system controldevice 2, meaning that if applicable, already-existing sub-routines ofother program units are accessed in order to keep the expenditure in theimplementation of the modules necessary according to the invention aslow as possible.

The method described in detail in the preceding as well as the describedmagnetic resonance system are only exemplary embodiments that can bemodified by those skilled in the art without departing from the scope ofthe invention. In particular other forms of excitation pulses orsequences of excitation pulses can be used instead of the excitationpulse concretely described. The invention was explained using in thecontext of a medically-utilized magnetic resonance system, but it is notlimited to such applications, and can also be used in scientific and/orindustrial applications.

Although modifications and changes may be suggested by those skilled inthe art, it is the intention of the inventor to embody within the patentwarranted hereon all changes and modifications as reasonably andproperly come within the scope of his contribution to the art.

1. A method to determine a flip angle distribution for each of aplurality of different antenna transmission configurations in aspecified volume in a subject in a magnetic resonance (MR) system havinga radio-frequency antenna with a plurality of resonator elements thatcan be activated individually or in groups in said plurality ofdifferent transmission configurations to generate a correspondingplurality of different, linearly-independent radio-frequency fielddistributions in an examination volume in which said subject isdisposed, said method comprising the steps of: implementing a pluralityof MR data acquisitions with a plurality of pulse sequences respectivelyusing different ones of said plurality of different transmissionconfigurations to emit respective radio-frequency signals that produce aflip angle in the specified volume, resulting in spatially-dependent MRsignals from said specified volume, and receiving said MR signals withidentical reception antenna configurations in each of said plurality ofMR data acquisitions; selecting said plurality of pulse sequences sothat, for each of said plurality of pulse sequences, a total function,which describes a dependency of the MR signal achieved at a specifiedlocation in said specified volume on the flip angle and on furtherMR-relevant parameters, can be factored into a first sub-function, thatis known, which describes said dependency of the MR signal on the flipangle, and a second sub-function that describes said dependency on saidfurther MR-relevant parameters; measuring an absolute flip angledistribution in said specified volume in said subject using one of saidplurality of different transmission configurations as a referenceconfiguration; and for each other transmission configuration in saidplurality of different transmission configurations, determining a flipangle distribution produced thereby from said absolute flip angledistribution of said reference configuration and from a ratio of saidspatially-dependent MR signals acquired in the respective MR dataacquisition to correspondingly spatially-dependent MR signals acquiredin said measurement using said reference configuration.
 2. A method asclaimed in claim 1 comprising employing a circularly polarizingtransmission antenna configuration as said one of said plurality ofdifferent transmission configurations used as said referenceconfiguration.
 3. A method as claimed in claim 1 wherein the MR signalsproduced using the respective plurality of different transmissionconfigurations have phasing associated therewith, and comprisingdetermining respective phasing for radio-frequency field distributions,produced in each of said plurality of MR data acquisitions, from thephasing of the MR signals.
 4. A method as claimed in claim 1 comprisingimplementing said plurality of MR data acquisitions respectively usingsaid plurality of different transmission configurations to cause, ineach of said plurality of MR data acquisitions, said flip angle to bewithin an unambiguous range of an inverse function of said firstsub-function.
 5. A method as claimed in claim 1 comprising employingmulti-pulse sequences as each of said plurality of pulse sequences and,in each of said multi-pulse sequences, emitting only one radio-frequencypulse with the respective, different ones of said plurality of differenttransmission configurations, and emitting all other pulses with saidreference configuration.
 6. A method as claimed in claim 5 wherein eachmulti-pulse sequence comprises an excitation pulse and a refocusingpulse, and emitting said excitation pulse with the respective, differentone of said plurality of different transmission configurations andemitting the refocusing pulse with said reference configuration.
 7. Amethod as claimed in claim 1 comprising implementing each of saidplurality of MR data acquisitions using an interleaved multi-sliceacquisition procedure comprising an excitation pulse and a refocusingpulse, and emitting one of said excitation pulse or said refocusingpulse slice-selectively and emitting the other of said excitation pulseor said refocusing pulse weakly slice-selectively.
 8. A method asclaimed in claim 7 comprising emitting said refocusing pulseslice-selectively.
 9. A method for generating a homogenousradio-frequency field distribution in a specified volume of a subject ina magnetic resonance (MR) system to determine a flip angle distributionfor each of a plurality of different antenna transmission configurationsin said specified volume, said MR system having a radio-frequencyantenna with a plurality of resonator elements that can be activatedindividually or in groups in a plurality of different transmissionconfigurations to generate a corresponding plurality of different,linearly-independent radio-frequency field distributions in anexamination volume in which said subject is disposed, said methodcomprising the steps of: implementing a plurality of MR dataacquisitions with a plurality of pulse sequences respectively usingdifferent ones of said plurality of different transmissionconfigurations to emit respective radio-frequency signals that produce aflip angle in the specified volume, resulting in spatially-dependent MRsignals from said specified volume, and receiving said MR signals withidentical reception antenna configurations in each of said plurality ofMR data acquisitions; selecting said plurality of pulse sequences sothat, for each of said plurality of pulse sequences, a total function,which describes a dependency of the MR signal achieved at a specifiedlocation in said specified volume on the flip angle and on furtherMR-relevant parameters, can be factored into a first sub-function, thatis known, which describes said dependency of the MR signal on the flipangle, and a second sub-function that describes said dependency on saidfurther MR-relevant parameters; measuring an absolute flip angledistribution in said specified volume in said subject using one of saidplurality of different transmission configurations as a referenceconfiguration; for each other transmission configuration in saidplurality of different transmission configurations, determining a flipangle distribution produced thereby from said absolute flip angledistribution of said reference configuration and from a ratio of saidspatially-dependent MR signals acquired in the respective MR dataacquisition to correspondingly spatially-dependent MR signals acquiredin said measurement using said reference configuration; and determiningoptimal transmission amplitudes for the respective plurality ofdifferent transmission configurations, for generating a homogenousfrequency field in said specified volume region, dependent on therespective determined flip angle distributions.
 10. A method as claimedin claim 9 wherein the MR signals produced using the respectiveplurality of different transmission configurations have phasingassociated therewith, and comprising determining respective phasing forradio-frequency field distributions, produced in each of said pluralityof magnetic resonance data acquisitions, from the phasing of the MRsignals, and determining optimal transmission phases for the respectiveoptimal transmission amplitudes dependent on the respective determinedphasing.
 11. A magnetic resonance (MR) system comprising an MR dataacquisition unit having a radio-frequency antenna with a plurality ofresonator elements that can be activated individually or in groups insaid plurality of different transmission configurations to generate acorresponding plurality of different, linearly-independentradio-frequency field distributions in an examination volume in which asubject is disposed; a system controller that operates said MR dataacquisition unit to implement a plurality of MR data acquisitions with aplurality of pulse sequences respectively using different ones of saidplurality of different transmission configurations to emit respectiveradio-frequency signals that produce a flip angle in the specifiedvolume, resulting in spatially-dependent MR signals from said specifiedvolume, and to receive said MR signals with identical reception antennaconfigurations in each of said plurality of MR data acquisitions; foreach of said plurality of pulse sequences, a total function, whichdescribes a dependency of the MR signal achieved at a specified locationin said specified volume on the flip angle and on further MR-relevantparameters, being factorable into a first sub-function, that is known,which describes said dependency of the MR signal on the flip angle, anda second sub-function that describes said dependency on said furtherMR-relevant parameters; a flip angle distribution measuring unit thatdetermines an absolute flip angle distribution in said specified volumein said subject using one of said plurality of different transmissionconfigurations as a reference configuration; and a flip anglecalculation unit that, for each other transmission configuration in saidplurality of different transmission configurations, determines a flipangle distribution produced thereby from said absolute flip angledistribution of said reference configuration and from a ratio of saidspatially-dependent MR signals acquired in the respective MR dataacquisition to correspondingly spatially-dependent MR signals acquiredin said measurement using said reference configuration.
 12. A magneticresonance system as claimed in claim 11 wherein the MR signals producedusing the respective plurality of different transmission configurationshave phasing associated therewith, and comprising a phase determinationunit that determines respective phase positions for radio-frequencyfield distributions, produced in each of said plurality of magneticresonance data acquisitions, from the phase positions of the MR signals.13. A magnetic resonance system as claimed in claim 11 comprising aparameter optimization unit that determines respective optimaltransmission amplitudes for the individual resonator elements, togenerate a homogenous radio-frequency field distribution in saidspecified volume, dependent on the respective determined flip angledistributions, and wherein said antenna activation device activates saidresonator elements in said plurality of different transmissionconfigurations to cause the respective individual resonator elements toradiate radio-frequency signals with said optimal transmissionamplitudes.
 14. A magnetic resonance system as claimed in claim 11wherein the MR signals produced using the respective plurality ofdifferent transmission configurations have phasing positions associatedtherewith, and comprising a phase determination unit that determinesrespective phasing for radio-frequency field distributions, produced ineach of said plurality of magnetic resonance data acquisitions, from thephasing of the MR signals, and comprising a parameter optimization unitthat determines respective optimal transmission amplitudes for theindividual resonator elements, to generate a homogenous radio-frequencyfield distribution in said specified volume, dependent on the respectivedetermined flip angle distributions, and wherein said antenna activationdevice activates said resonator elements in said plurality of differenttransmission configurations to cause the respective individual resonatorelements to radiate radio-frequency signals with said optimaltransmission amplitudes.
 15. A computer-readable medium encoded with adata structure to determine a flip angle distribution for each of aplurality of different antenna transmission configurations in aspecified volume in a subject in a magnetic resonance (MR) system havinga radio-frequency antenna with a plurality of resonator elements thatcan be activated individually or in groups in said plurality ofdifferent transmission configurations to generate a correspondingplurality of different, linearly-independent radio-frequency fielddistributions in an examination volume in which said subject isdisposed, said data structure, when loaded into a controller of saidmagnetic resonance system, causing said magnetic resonance system to:implement a plurality of MR data acquisitions with a plurality of pulsesequences respectively using different ones of said plurality ofdifferent transmission configurations to emit respective radio-frequencysignals that produce a flip angle in the specified volume, resulting inspatially-dependent MR signals from said specified volume, and receivingsaid MR signals with identical reception antenna configurations in eachof said plurality of MR data acquisitions; for each of said plurality ofpulse sequences, a total function, which describes a dependency of theMR signal achieved at a specified location in said specified volume onthe flip angle and on further MR-relevant parameters, being factorableinto a first sub-function, that is known, which describes saiddependency of the MR signal on the flip angle and a second sub-functionthat describes said dependency on said further MR-relevant parameters;measure an absolute flip angle distribution in said specified volume insaid subject using one of said plurality of different transmissionconfigurations as a reference configuration; and for each othertransmission configuration in said plurality of different transmissionconfigurations, determine a flip angle distribution produced therebyfrom said absolute flip angle distribution of said referenceconfiguration and from a ratio of said spatially-dependent MR signalsacquired in the respective MR data acquisition to correspondinglyspatially-dependent MR signals acquired in said measurement using saidreference configuration.